Aluminum foil is commonly fabricated from aluminum ingots, which are heated and compressively rolled into thin metal leaves having a general thickness range of a few thousandths to a few tenths of a millimeter. This process affects the microstructure of the metal, for example by pulling the axis of the aluminum crystallites, or grains, in the direction in which the metal is rolled. The resulting grain deformation and weakened microstructure of the metal from repeated application of compressive rolling forces fatigues the aluminum and leaves it susceptible to both intergranular and extragranular corrosion. In particular, the surface of aluminum foil produced in the traditional manner is replete with surface voids, microstructural gaps, and other irregularities, all of which represent potential corrosion sites. Indeed, foil thinner than about 0.025 mm may even have minute pinholes caused by the production process (as a reference, most aluminum foils produced for culinary use are between about 0.010 and 0.030 mm thick).
The end result of a corrosion reaction involves a metal atom being oxidized, whereby it loses one or more electrons and leaves the bulk metal. Metals may (and frequently do) corrode on contact with water and/or ambient moisture, acids, bases, salts, various gases, and some oils. Many of these substances are contained in various foods.
if left in an environment with sufficient ambient moisture, the surface of raw aluminum foil will typically oxidize and form a thin layer of aluminum oxide. Although this layer may protect the foil from further corrosion, it also may react with acids to form aluminum salts. Corrosion and other reactions may be enhanced in the presence of directly or indirectly applied heat, such as in cooking applications.
Corrosion and other chemical reactions resulting in the formation of aluminum compounds is a problem in culinary applications for many reasons. One problem is the formation of corrosion and other reaction byproducts and subsequent deposit on food with which aluminum foil is used. Depending on the corrosive agent, the departing metal atoms, referred to herein as radicals, may be in the form of different compounds. For example, when aluminum foil comes into contact with food ingredients such as salt, or vinegar, or acidic foods such as tomatoes, aluminum salts form and deposits on the food.
Another problem is mechanical disintegration. The corrosion process may itself leave small holes in the foil, making it susceptible to tearing. In addition, mechanical stress to aluminum foil in many culinary applications, for example from food adhering to the foil surface, and/or abrasion from utensils, may also result in tearing the foil, and possibly transferring bits of the foil to the food during use.
Although the toxicity of aluminum and aluminum salts is generally not considered to be harmful to healthy humans in small quantities, many studies indicate that aluminum and aluminum-containing compounds are not particularly beneficial. For example, aluminum toxicity is recognized in many settings where exposure is heavy or prolonged, or in certain medical conditions such as limited renal function. There are over 2000 references in the National Library of Medicine on adverse effects of aluminum. Regardless of toxicity concerns, however, it is believed that many individuals would prefer not to have to remove conspicuous aluminum foil reaction products, and/or bits of degraded aluminum foil, from their food before eating it.
One approach to the corrosion problem include anodizing aluminum foil, but this process is generally unsuitable for culinary applications: the anodization process typically requires a sealing process that uses toxic compounds (e.g. nickel salts or dichromates) and is otherwise ineffective to block aluminum radical formation. As such, most culinary aluminum foil is raw (not anodized). Another approach is the use of aluminum-based alloys in order to reduce porosity, such as the process disclosed in U.S. Pat. No. 5,466,312 to Ward, but even if this method decreases the amount of potential corrosion sites, it likewise does not block aluminum radical formation at the remaining corrosion sites. Recent approaches involve applying a non-stick coating to culinary aluminum foil, such as U.S. Pat. No. 6,423,417, 6,544,658, and 6,696,511, all to Robbins, but the disclosed methods generally involve toxic curing agents and require additional process steps to properly cure the coating, and do not address the underlying problem of preventing corrosion from occurring. Further, the disclosed coatings are either silicone-based, or polymers, neither of which has a strong commercial appeal among consumers who prefer using natural lipid-based products, such as vegetable cooking oils, for lubricity.
The disclosures of the patents mentioned above are incorporated herein by reference.